So JRR Tolkien walks into the HBO meeting and pitches his great new fantasy idea: "It's set long ago, and there's this magic ring, and some hobbits – they're like organic dwarves – and there's this wizard with a long white beard, and, and …" Four hours later he gets to the end of his summary. "Not bad," says one executive, "but what about the second season?"

"And there's no sex in it at all!" says another.

"And the language is kind of tame, why don't the characters swear more?"

"And what about the violence? Couldn't we put in a few more beheadings, spurting arcs of arterial blood, that sort of thing?"

"And all this 'good' and 'evil'. It's a bit black and white, isn't it?"

"How about some racier themes? You know, like extortion or prostitution, or … I know: incest!"

Fantasy is not a genre you would ever expect to describe as having "grown up", but let's at least say it's moved on since Tolkien's day. If The Lord Of The Rings is like the gateway drug of high fantasy, then today's fans crave something harder. They could swallow armies of talking trees, indestructible wizards and fire-breathing balrogs, but a mythical realm where the nearest thing to a sexual encounter is a bromance between two hobbits? Come on, JRR!

But while it was never much of a stretch to inject more sex and violence into the fantasy genre, the result has invariably been adolescent pulp fiction with over-compensatory weaponry and dodgy cover art, rather than classic literature. There's a dilemma here: modern fantasy fans don't just want the cheap rush of "adult" content, they also want the lasting satisfaction of believable characters, absorbing plots, narrative refinement. Like a mix of crack cocaine and fine wine.

A similar split runs through the world of fantasy movies. Occasionally an audience of well-adjusted civilians can be attracted to a marquee fantasy franchise, like Peter Jackson's Lord Of The Rings movies, but more often than not, they're too coarse or silly for all but the hardcore junkies. Many recent attempts have either dished out too much sex and violence – Ironclad, Season Of The Witch, Black Death – or watered everything down in search of a juvenile audience (the Narnia films, The Golden Compass, Percy Jackson). What mystical alchemy could ever resolve these infernal dichotomies?

At this point, like a rampant elf lord responding to the horn of Gondor, enter Game Of Thrones, HBO's hugely expensive and ambitious new fantasy series. Thrones has two things which could help it unite fantasy's beleaguered tribes: first, it is based on the novels of George RR Martin, which are about as mature as fantasy literature gets in both senses of the word; secondly, it's on television. Yes, television! In the past, small-screen fantasy meant Saturday teatime fare like Xena: Warrior Princess and Hercules, but HBO has seen how Peter Jackson did it and realised it's got the budgets, the stars, and the mature viewers to go one better. What's more, it's not limited by feature length. The 10-part first season of Game Of Thrones covers just one of the seven Tolkien-sized volumes in George RR Martin's A Song Of Ice And Fire series, which he's been writing for the past 20 years (volume five is out this summer). Movie studios have tried to buy the rights to Martin's books in the past but he refused.

'I knew it couldn't be done in a film - the scope was too big. The only way to do this series was with HBO' - writer George RR Martin

Tyrion "The Imp" Lannister. Photograph: HBO

"I knew [it] couldn't be done in a film – the scope was too big," he says. Nor could Thrones have been done on regular television, given its adult content: "The only way to do this series was with HBO." A new era of epicness beckons.

On the face of it, Thrones follows the established fantasy rules: an imaginary realm requiring a big map (the continent of Westeros), a northern European medieval vibe, funny character names (Daenerys Targaryen, Khal Drogo, er, Jon Snow), and an almighty power struggle. But the "we're not in Middle Earth now" message is rammed home in the opening episode by way of two graphic beheadings, a scattering of severed limbs, some softcore disrobing, a decadent dwarf cavorting with prostitutes, incestuous relations in a turret, and the king of Westeros declaring, "All I wanted to do was crack skulls and fuck girls." Put that in your pipe, Gandalf!

Rather than a gratuitously amped-up spectacle, HBO would prefer you to think of Thrones as "The Sopranos in Middle Earth". The tag fits in the sense that there are no clear-cut goodies and baddies and no character is too big to be medievally whacked at the drop of a gauntlet. On the other hand, Thrones' feudal society doesn't leave much scope for social commentary or progressive values, and although the story is almost Wire-like in its complexity, with dozens of characters and multiple subplots to keep track of, many of them wouldn't look out of place in a Greek tragedy or even a 1980s soap opera.

If there's a central character to Game Of Thrones, it's Eddard Stark, lord of the northern realm of Winterfell and best mate of the king. He's played by LOTRs' Boromir himself, Sean Bean.

"He's got the world on his shoulders," says Bean of his character, "But he's no squeaky clean hero. He's a very loyal man, very honorable and truthful, but he's been around. He's had at least one affair, and his wife gets a bit prickly when the subject of his bastard son comes up, which it does quite a lot."

Bean, of course, lends the show a little of that movie fantasy pedigree, having been a veteran of not just Lord Of The Rings but also Troy, Black Death, and others. He rates Thrones a comparable production to Lord Of The Rings in terms of scale. Rather than New Zealand, it makes use of Northern Ireland and Malta, augmenting existing castles with huge sets built in quarries and a disused Belfast shipyard.

'You think of fantasy and you imagine it's going to be enchanting … but this has got that edge. It's sexy, it's brutal, it's horrific, it's moving' - Sean Bean

Daenerys Targaryen. Photograph: HBO

The difference in tone from Tolkien's world is stark, says Bean: "You think of fantasy and you imagine it's going to be enchanting rather than disturbing, but this has got that edge to it. It's sexy, it's brutal, it's horrific, it's moving, but it's for a good reason. The characters are so poisonous. The monsters are the people!"

If it works, Game Of Thrones Thrones could broker a lasting and lucrative peace between the legions of fantasy geekdom and the aloof tribes who worship at the altar of the HBO box set. But Thrones is not alone in storming the portcullis of the fantasy entertainment kingdom. Oh no.

From HBO's upstart rival Starz comes Spartacus, an altogether more barbaric and carnal challenger. If Thrones' movie model was Lord Of The Rings then Spartacus' would be Tinto Brass's Caligula, with a touch of Zack Snyder's 300. Here is a Roman world of cartoonishly bloody gore, naked flesh used as set dressing, and dialogue combining cod-Latin and geezer gangster: "You counsel to suck the cock that pisses on me!". The sex and violence isn't just gratuitous in Spartacus. It is Spartacus, which has made it something of a guilty pleasure on both sides of the Atlantic. And pointing up just how far small-screen fantasy has come, Spartacus' lead imperatrix is none other than Lucy Lawless, who sheds a lot more than just her wholesome Xena image. If that doesn't work, Starz is marshalling, undressing and butchering more familiar Brit stars in Camelot, which looks like an emo-flavoured retelling of King Arthur. Coming to Channel 4 in the summer, it features Jamie Campbell Bower, a bit-player in the Twilight and Harry Potter movies, as the youthful new king, Eva Green and Joseph Fiennes are also in there. By now you know what to expect: swords, sorcery, sweeping vistas and, yes, sex.

The Arthurian connection reminds this writer of the last age of adult fantasy. John Boorman's 1980s epic Excalibur struck a similar balance with its lusty battles, sex in armour and straight-faced mysticism, all girded by an earthy realism. There was also Paul Verhoeven's characteristically uninhibited and graphically titled Flesh+Blood, with Rutger Hauer and Jennifer Jason Leigh pillaging and shagging their way through 16th-century Europe. And, of course, there was the red meat of Arnold Schwarzenegger's Conan/Red Sonja films. Perhaps the makers of Game Of Thrones realise what John Boorman and co did: that sex and violence (and perhaps a small dose of politics) are essential if anyone's going to take this stuff seriously.

If Game Of Thrones and its small-screen challengers do conquer fantasy, where will that leave the movies? Well, returning to the 80s. Later in the year, a new Conan flexes his pecs: brutish Hawaiian beefcake Jason Momoa, who, coincidentally, plays Khal Drogo in Game Of Thrones. A new Red Sonja has been rumoured for years, while Guy Ritchie is reportedly sniffing around an Excalibur remake. And let's not forget about that little thing going on down in Middle Earth itself. How will The Hobbit's epically delayed but relatively gentle adventuring compete in this new steroid-pumped market? If Peter Jackson has any sense, he'll be inserting some extra scenes of gory, incestuous dwarf sex even as we speak.

GAME OF THRONES: A BEGINNER'S GUIDE

HOUSE STARK The gruff Starks of the north are reluctant participants in the shifty politics of King's Landing, the southern capital. John Prescott in furs, basically …

HOUSE BARATHEON King Robert is the current warmer of the spiky Iron Throne, though he'd rather be out drinking, hunting and swiving than defending his realm.

HOUSE TARGARYEN The former royal family, exiled after King Robert's rebellion. Prince Viserys offers sister Daenerys to the Dothraki warlord as a bribe for his horde.

HOUSE LANNISTER Locked in a loveless marriage with King Robert, Queen Cersei and her brothers Jaime and Tyrion (The Imp) seem to be playing the long game here.

THE DOTHRAKI Nomadic warriors who like horses, pillaging, long hair and more pillaging. House Baratheon hope the Dothraki fear of water will keep King's Landing safe.

• Discuss the show with us every Monday night on our Game of Thrones series blog, launching after the first episode